1. Field of the Invention
The present invention pertains to a fuse for insertion in the primary circuit of a high voltage transformer or other device and having a rupturable element wire which melts at a predetermined current level. The fuse includes an arc intercepting element and a resistor in circuit with the intercepting element to reduce fault currents momentarily to an arc extinguishing level to clear the circuit.
2. Background Art
In the art of protective fuses for electrical circuits and the like, and in particular for high voltage transformer circuits, it is known to provide so-called protective links to remove an internally faulted transformer from the primary line thereby preventing outages to other circuits on the line not served by the faulted transformer. The conventional approach to providing circuit clearing fuses for faulted transformers and other high voltage circuits has included the provision of a fuse having a component which melts or decomposes to produce an arc extinguishing gas to eliminate continued arcing. However, the conventional silver/sand current limiting fuses are expensive and it has been impractical to equip distribution transformers with this type of fuse. At the same time, there has been an increasing number of systems wherein potential fault currents are much higher than previous fuse designs are capable of interrupting without some danger to equipment and to the environment surrounding the transformer itself.
It has been determined that conventional weak link type current limiting fuses which, for example, operate by utilizing gas pressure to propel one end of the fuse structure away from the other end in an oil filled transformer to provide circuit interruption, have been found to be suitable for fault currents in the range of 1500 amps or less. For higher fault currents it has become necessary to strengthen the mechanical structure of the fuse, as well as the support bushing for the fuse. This has become increasingly expensive and a not entirely satisfactory solution to failures wherein fault currents in the range of 3,000 to 35,000 amps may occur.
Accordingly, since the conventional rupturable element type fuse, provided with a material such as a vulcanized fiber tube surrounding the fuse element to provide the arc extinguishing gas, has been relatively successful for current levels in the range of 100 to 1500 amps, it has been determined that it is desirable to provide for a fuse structure which will insert a resistor in the circuit on failure of either the transformer or any other circuit which might occur within the transformer enclosure. The object of the present invention is to provide for directing the arc established upon melting of the fuse element in such a way that it passes through a resistor to at least momentarily reduce the current level to a value which will result in extinguishment of the arc in the presence of a deionizing gas. This functional advantage is provided by structure which has heretofore not been provided in the art of protective link fuses for transformers and the like. Moreover, the particular arrangement of a protective fuse in accordance with the present invention also provides, in one compact structural unit, the capability of protecting the circuit against short circuit conditions when the current is at a relatively low level and upon heating of the resistor whereby sufficient mechanical separation of the fuse conductor elements may be obtained to prevent arc establishment or restrike.